1. Field of the Invention
The present invention relates to a motor driving device suitably used to assist driving torque of an automobile by using a motor as in the case of a hybrid car, and to an automobile using the motor driving device. More particularly, the present invention relates to a motor driving device suitable for driving a motor with a doubling winding structure, and to an automobile using the motor driving device.
2. Description of the Related Art
Recently, in the industrial field of automobiles, importance has been laid in improving fuel economy and cleaning exhaust gas from the viewpoint of adaptation for environmental issues, and attention has been focused on a hybrid car using a motor to assist torque in a speed range where fuel economy efficiency of an engine is not so high. A motor is superior in torque response to an engine and is expected to not only improve fuel economy, but also to serve as a useful component in running control, such as VDC (Vehicle Dynamic Control), in future. The motor is constituted by a permanent-magnet type synchronous motor with high efficiency and is used at high speed range in which a maximum rotation speed exceeds 10,000 rpm. In the motor, an induced voltage (also called counter electromotive voltage) is generated in proportion to the rotation speed, and a current is caused to flow due to the difference between the power supply voltage and the induced voltage. At the high speed in excess of 10,000 rpm, it becomes hard to flow a current due to an influence of the induced voltage. Therefore, a method of suppressing the induced voltage by the so-called field weakening control is employed. Lately, a method of boosting a battery voltage is also employed.
The method of boosting the battery voltage is performed by providing a boosting converter between a battery and an inverter, and by repeating operations of causing a battery current to temporarily flow through a reactor to be stored therein as electromagnetic energy, and supplying electric power to the inverter with the reactor used a current source. However, the reactor adaptable for an output in excess of several tens kW is increased in both size and weight. Accordingly, there is a demand for a boosting method which requires a smaller reactor or uses no reactor.
As a first known example of the boosting method using no reactor, Patent Document 1 (JP-A-7-115704), for example, discloses a method of using a double-winding motor and two inverters. According to the disclosed method, a high-voltage winding and a low-voltage winding are wound around a motor stator to constitute one kind of transformer. The motor itself is driven by a first inverter connected to the high-voltage winding. When the voltage of a low-voltage power supply is boosted, the stator is excited by a second inverter connected to the low-voltage winding, and a current is rectified by the first inverter and is charged in a high-voltage power supply.
As a second known example, there is a method described in Non-Patent Document 1 (“Novel Power Conversion System for Cost Reduction in Vehicles with 42V/14V Power Supply” (SAE 2003-01-0307)). According to the described method, a motor has a structure of a single high-voltage winding, and a low-voltage power supply is connected to a neutral point of the motor. A motor winding is used instead of a reactor by utilizing the operation that an inverter connected to a high-voltage winding is turned on and off with PWM (Pulse Width Modulation) control.